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Redox and ORP

By Eric Edelman. Presented 08/03/1997 on #reefs IRC.

OK, you've got a stable tank, your pH, alkalinity, and Calcium levels all check out. Your corals are doing well, and coralline algae is starting to grow. Then some wiseass asks you, "What's your ORP readings?" Your day is shot. You feel like you've missed out on something important.

ORP? What's ORP?

ORP is short for oxidation - reduction potential. It's companion term is Redox. Redox is a chemical term referring to an oxidation-reduction reaction. These guys work as a team, so one can't happen without the other one.

What I'm going to do tonight is give you an explanation of what these terms mean, how the reactions work, and what's going on with your tank. Then we'll go into the practical aspects of it with your tank, and you'll learn how to play with your ORP.

Since we're going to be using a few chemical reactions to illustrate what I'm going to tell you, I'd like everyone to fire up their web browser and point it to: http://207.36.44.119/example.html I can't type in subscripts/superscripts here, so this seemed easier.

Let's talk about what a REDOX reaction is first. A REDOX reaction has two parts.

  • Oxidation - an atom loses one or more electrons, he's just been oxidized, and his oxidation number increases
  • Reduction - an atom gains one or more electrons, this guy's been reduced, and his oxidation number decreases

Sounds backwards doesn't it because the guy who's reduced is actually gaining electrons? Well, what we're really looking at is the fact that his OXIDATION NUMBER has been lowered. Take a look at equation #1.

What we have here are two atoms of Sodium (Na) a nice chrystalline solid, and two atoms of Chlorine (Cl) which make for a lovely, yet pungent, diatomic gas. In their elemantal state (not bound to anything else) these atoms have oxidation numbers of 0. You can see the superscript 0's in the equation. Once the reaction has been completed, sodium has been oxidized, chlorine has been reduced. Below the reaction you will see a summary of what has occured.

Equation #2 is very similar, but it does illustrate that you do not have to start with atoms with zero oxidation numbers. Well Gee, that's great. What's it got to do with my Fish Tank? Good question.

REDOX reactions are going on in your fish tank all the time. Many different types of reactions such as synthesis, addition, combustion, and decomposition reactions can all be additionally classified as REDOX Reactions.

It also happens that most of the common elements and compounds that are in our tanks such as calcium, sodium, chlorine, bromine, etc... all undergo these type of reactions. Many of these reactions provide our reef inhabitants with the different compounds they need to grow and live.

Now the good part about all these reactions is that they are highly ionic in nature. This means that the reaction involves a transfer of electrons from one atom to another in order to create or break a bond.

That's the big mystery behind the REDOX reaction.

So what's ORP?

ORP is Oxidation - Reduction Potential. Completely simplified, it is the likelihood of, or driving force behind, these REDOX reactions occuring. It's important in our aquariums because this is what we are measuring when we are talking about ORP values. We measure ORP in milliVolts (mV).

And now on to the practical aspects of ORP.

The first question usually asked is, "What should my ORP values be?" The answer is that they shouldn't really BE anything. ORP values vary from tank to tank, and they even vary within one single tank.

The best indication ORP is the trend that they have, that is where all the important obtainable information is. Some general ideas, or rule of thumb, on where your ORP value should be. 200 mV is pretty low. 500 mV is pretty high. My own personal tank ran at around 375 in midafternoon. When it dropped to 325 or so, I'd clean the skimmer out, and the ORP would rise again in a few hours. Good skimming is the BEST way to maintain solid ORP values. ORP correlates with dissolved oxygen, and inversely correlates with your pH.

  • Things that lower ORP: Dysfunctional Skimmer, increased biological reactions, poor gas exchange.
  • Things that raise ORP: Good skimming, a few chemicals, good gas exchange, photosynthesis.

One general trend to look at is that ORP rises when the water becomes rich in oxygen. We can enrich our tanks with oxygen through vigorous protein skimming and photosynthesis. Those are relatively "natural ways" to increase your ORP value.

Now, on to the Big 3: Hydrogen Peroxide, Potassium Permanganate, and Ozone.

Below the equations you should have in your browser window, I've listed the chemical formulas of all three of these guys. In addition, you can see how ozone is actually formed. Hydrogen Peroxide, Potassium Permanganate, and Ozone are all strong oxidizing agents. By adding these guys to your tank, you can increase your ORP. Hydrogen Peroxide and Potassium Permanganate are liquids that can be added dropwise to your tank. Ozone is a electrically generated gas.

IMPORTANT: If you're going to play with one of these 3 chemicals, it is HIGHLY advisable that you have a functioning ORP Meter on your tank. And you monitor that meter closely. Blasting your ORP value up can be just as bad, if not worse than having a tank with low ORP. That being said, usual dosages of Hydrogen Peroxide are a drop or two per gallon, Potassium Permanganate is dosed somewhere around a teaspoon to a tablespoon per 50 gallons of water. You can get Hydrogen Peroxide at the drug store. Potassium Permanganate is available from chemical supply houses, Sears sells it as a water softener germicide, and Albert Thiel sells it under the name Redox +. Ultralife, Kent, and Coralife also bottle it for aquariums, but I don't know the trade names. It may be available from other manufacturers as well. Cheapest source is probably Sears.

Ozone is a completely different story. Ozone you have to make yourself with an Ozone Generator. Ozone used to be pretty popular for saltwater fish and reef tanks a few years ago. My gut feeling on that is that it was popular because we didn't have the availability of good quality skimmers like we do today.

It's still a popular thing for very large systems as well. Today, you are far better off buying a $500 skimmer than you are buying a $200 skimmer and a $300 ozone generator/controller combo.

Ozone is pretty dangerous stuff if it's used incorrectly. It's a blue tinted gas at room temperature, and if you breathe it, you'll get a bad headache. If you breathe enough of it, it'll probably kill you. To use it safely, run it through your skimmer and the air vent from the skimmer through activated carbon. This means neoprene tubing and plastics that are ozone safe.

Or, you can run it through a special ozone reactor. Ozone reactors basically mix the ozone with your tank water, the idea being it's more efficient to do it this way than to run it through your skimmer. Any escaping ozone MUST be run through an activated charcoal filter so it cannot escape into the air.

In todays world of reefkeeping, we've pretty much gotten away from ozone usage. If you feel some burning desire to use ozone, there are a few products on the market that combine an ozone generator with an ORP meter and controller. Red Sea makes a nice one, but it's sort of pricey. You could also go with a manually adjustable ozone generator and a monitor, but that means you really need to be watching that monitor. Ozone can jack up your ORP values pretty fast, so this is more risky.

The more common use of these is a temporary quick fix for a problem. Say your tank is kind of polluted with DOC's, and you've got a nice little crop of Red Slime Algae growing. Any of these three will bust up some of the DOC's so your skimmer can pull them out of the water. When the problems gone, you quit using them.

Want to add a bunch of animals at once and try to compensate for the increased bioload? You can try doing that with an oxidizing agent as well.

But, in my opinion, to use one of these three methods on a regular basis is asking for trouble. As a quick fix, sure, go ahead. But not forever.

So to conclude.

We've covered what a REDOX reaction is, and why it's important. We've also looked at what ORP is and what values you should be looking for. Finally, we've discussed ways to change your ORP and why you'd want to do it. Thanks for listening. At this point, I'm ready for any questions you may have. Let's stick to

ORP/REDOX questions first, and then anything else you may want to ask.

<Saltman> Question: If you ORP values vary so why bother with it? Isnt it just one more thing to worry about? Wouldnt good maintenence and husbandry practices make more sense than Ozone or dumping hydrdgen peroxide in your tank?

<EricEE> I'm not recommending dumping ozone or hydrogen peroxide in the tank as a maintenance measure.

<EricEE> What I am recommending is that if you have a serious ORP problem (low ORP) and it's due to something that you can't pinpoint like a dead, hidden animal, using an oxidant to remove the waste is an option.

<EricEE> For example, snails have pretty high mortality rates and sometimes croak in the darndest places. If you can't find the dead animal and it's decaying, this is a good quick fix.

<Saltman> Question: what is the best ORP monitor for a 50 gal reef?

<EricEE> Best is probably a unit from Cole Parmer, but it's very expensive.

<EricEE> Good would be a PinPoint Brand (around $100) or if you are using a Neptune or Octopus Controller, they also monitor ORP.

<Saltman> Question: Aren't ORP meters and controllers just another expensive toy that manufactures want you to spend all your coral buying money on?

<EricEE> Good question.

<EricEE> If you were ever going to try ozone, hydrogen peroxide, or a permanganate, it's an essential piece of equipment.

<EricEE> Watching the general trend of your ORP readings can give you some additional information on how your tank is doing also.

<EricEE> Personally, I think it's a useful piece of equipment. Not essential.

<Saltman> Question: Is ORP temperature dependent, or salinity dependent?

<EricEE> salinity since it's an electrochemical process

<EricEE> but, I wouldn't worry about an erroneous reading of ORP due to water evaporation. It's not that dependant.

<Saltman> Question: Is there a level that is considered to high, and if so how do you get it down?

<EricEE> The only way you're going to get a level that's too high is by dumping ozone or oxidizing agents into the tank.

<EricEE> 500 is probably too high, and the corals will get irritated and close.

<EricEE> If you overdose ozone, you can use hydrogen peroxide to react with the ozone and lower the ORP

<Saltman> Question: You see your ORP measurements going down. What do you do?

<EricEE> Try and figure out why.

<EricEE> Clean your skimmer, see if it goes up.

<EricEE> If it doesn't go up, look for something that might have died and is now decaying in the tank. That's the most common cause of a drop in ORP.

<EricEE> ORP will naturally drop when you add animals and some additives.

<EricEE> 10mV drop is nothing to worry about. 50-100 is something to take seriously.

<Saltman> Question: can you give a more indepth explaniation of hydrogen peroxide use,i.e.a more chemistry related explanation,and weather or not its use would impact other smaller lifeforms benificial to the reef tank?

<EricEE> OK, H2O2 dissociates in water, releasing a hydrogen

<EricEE> and oxygen gas

<EricEE> when it does this, it removes the hydrogens from water molecules and their electrons

<EricEE> which leaves you with two negatively charged oxygen ions which are highly reactive

<EricEE> This happens very quickly, so almost immediately, the charged oxygens react with something else, hopefully some piece of pollution (DOC) which is then more easily skimmed out.

<EricEE> So the overall reaction is:

<EricEE> 2 H2O2 + 2H+ (from water) + 2e- (from water's oxygen) -----> 2 H2O

<Saltman> Question: How do you know if you've got "an erroneous ORP reading"?

<EricEE> Erroneous ORP readings are actually pretty easy to get. For example, if you are using a PinPoint meter, it's not adjustable, and the probe is not the best quality out there.

<EricEE> So if your tank looks fine and your skimmer is cranking away and your ORP meter says 120, I'd look at the meter first.

<EricEE> Oh, back to the equation I posted a moment ago:

<EricEE> 2 H2O2 + 2H+ (from water) + 2e- (from water's oxygen) -----> 2 H2O

<EricEE> The electrons are more likely come from the DOC's that are oxidized during the reaction, not the water

<Saltman> Question: I was told that dd skimmers did a good job of oxygenating the water thereby taking a lot of the worry about ORP away. How much can a dd affect the ORP ?

<EricEE> If you're looking for me to say, "It'll raise your ORP from 350 to 400", I can't say that.

<EricEE> But, the larger downdrafts do indeed oxygenate the water better than many of the CC and venturi units on the market.

<Saltman> Question: Is there a significant difference on coral growth if redox is 150 as opposed to 350

<EricEE> If your ORP is 150 you probably won't see any growth since your water will be oxygen poor.

<EricEE> Most tanks should run somewhere around 300-400.

<Saltman> Question: My make up water is from the local grocery store - bottled ro water which the label says has been ozonated. Do I need to worry that my ORP is going up?

<EricEE> No, when they treat the water with ozone they will be using something to remove the ozone.

<EricEE> You won't have any residual ozone in the water.

<Saltman> Question: (2 parts) part 1, How quickly does the ORP number respond when you adjust tank conditions? Is it immediate?

<EricEE> If you add a bunch of new animals into the tank, you'll see your ORP drop very soon. Now how much it's going to drop is going to depend on how much of an overall change in bioload there is.

<EricEE> If something dies and decays, you'll see a slow downward trend.

<EricEE> If you add chemicals/ozone, you'll see it go up withing a few minutes.

<Saltman> Question: (2 parts) part 2, If redox is the "potential for ionic exchange" are there good exchanges and bad ones...in other words, does the redox number identify both good and bad potential exchanges?

<EricEE> The potential exchanges aren't really "Good" or "Bad" they're just chemical reactions that occur.

<EricEE> So yes, the REDOX potential will affect ANY electrochemical reaction, which in a reef tank, covers most of the reactions that occur.

<Saltman> Question: How do you clean an electrode properly?

<EricEE> If it's crusty, clean it with a soft toothbrush, make sure you keep it in the water while you're doing it though, the bulb on the end has to stay immersed in order to function properly. You can ruin the probe if you let it dry out.

<Saltman> Question: are large fluxations say between 275 and 400 dangerous to corals?

<EricEE> 275 to 400 is a pretty big swing. If it happens in an hour, it probably isn't too good. If it happens over the course of a week, it's probably fine. Keep in mind that something had to happen in order to shit your ORP 125 mV, so I'd try and find out what did that.

<Saltman> Question: If your tank looks fine and your skimmer is cranking and all you other params are good why worry about ORP

<EricEE> Same reason we worry about pH, alkalinity, and calcium levels I'd imagine. To make sure everything is withing reasonable parameters.

<Saltman> Question: can you explain a staghorn growth of 2 cm a month with redox of 125?

<EricEE> Oh, I meant shift a few lines above there.

<EricEE> woops

<EricEE> 2 cm a month is pretty astounding growth rate to begin with.

<EricEE> I'd have to say either your probe or meter isn't working correctly. 125 is a very low ORP.

<EricEE> Can I have a fragment of it though?

<Saltman> Question: You have said that the Pinpoint monitor is not all that good, do you recommend a brand?

<EricEE> If you're serious about measuring ORP, the Pinpoint monitor is hobbyist grade. Cole Parmer makes similar equipment, but using a better quality probe and a better quality, adjustable meter. But, you will pay for it. For $100 the Pinpoint unit's not bad.

<EricEE> My biggest fault on the PinPoint meter is that it's not adjustable.

<Saltman> Question: I just got some kmno4 crystals. I was wondering how to dilute and how much to add to my tank.

<EricEE> Hmm good question

<EricEE> KMnO4 has a molecular mass of 158 g/mol

<EricEE> So if you have 158 g of pure KMnO4 and you put it in a liter of water, you'll get a 1 molar solution.

<EricEE> I don't know the molarity of the commercially available stuff.

<Saltman> Question: Do the 2-part buffers like B-ionic effect ORP?

<EricEE> Maybe a little bit when you first add them. You're dissolving a bunch of salts in water, that should swing ORP a little, but not much.

<EricEE> Biggest effect on ORP is going to be how much oxygen your skimmer is putting into the water and how much photosythesis is going on.

<EricEE> OK, Craig Bingman recommends a 1% solution of KMnO4. So, if you add 1.58 g of it to 1 liter of water, that should work.

<Saltman> Question: What should my ORP number be?

<EricEE> I think I already covered that in the talk. Didn't you pay attention

<EricEE> Seriously, there's no absolute good or bad value for ORP

<EricEE> 100 is very low

<EricEE> 500 is very high

<EricEE> you'll have problems with each of those.

<EricEE> 300 is fine, 400 is fine

<EricEE> Just kidding about the paying attention part!

<EricEE> :)

<Saltman> Question: how can you be sure there is no resudal ozone in your tank?

<EricEE> Ozone is very reactive so it's not going to be around a while.

<EricEE> If you use carbon in your tank, that will take care of any that remains in the water.

<EricEE> More importantly is ozone escaping into the environment.

<DBW> OK people, just an important note on ozone...

<DBW> this is a mean little molecule and does major damage to living tissue

<Saltman> Question: with kmno4 oxidize (br) into something toxic, like ozone will.

<DBW> as a result you don't want some getting into your water or into the air...

<DBW> therefore where every ozone is used make sure that there is activated carbon...

<DBW> on all lines leaving the unit...

<DBW> i.e. air and water.

Created by liquid
Last modified 2006-11-26 13:55
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